Anatomy and Physiology of Children and Adolescents - M. R. Sapin 2007

Structure of the Human Body
Tissues, Organs, Organ Systems, and Apparatuses
Muscle Tissues

Muscle Tissues include striated (skeletal), non-striated (smooth), and cardiac muscle. These types of Muscle tissue differ in origin and Structure, yet they share common Structural and functional features—specifically, The ability to contract, change length, and shorten.

Striated (skeletal) muscle tissue forms the Muscles Attached to the skeletal bones. Upon contraction (shortening) of skeletal muscles—whose Functions are under conscious voluntary control—the bones (acting as levers) perform targeted movements. Cytology/practical/58.html">Striated Skeletal Muscle is formed by muscle fibers that can reach 10—12 cm in length in certain muscles. Externally, each muscle fiber is encased in a membrane called the sarcolemma, into which fine Collagen fibers known as the endomysium are interwoven. Beneath the sarcolemma, within the Cytoplasm (sarcoplasm) of each muscle fiber, lie numerous nuclei (up to 100), specialized Organelles (myofibrils), general-purpose organelles, and inclusions (Myoglobin, Glycogen). Dissolved in the sarcoplasm, myoglobin is a pigment-containing protein structurally similar to erythrocyte Hemoglobin, which gives muscles their red color.

The bulk of a muscle fiber consists of specialized organelles called myofibrils (Fig. 12). Myofibrils are formed by filaments of the contractile Proteins Myosin and Actin, arranged in a specific orderly fashion along the muscle fiber. These protein filaments (myofilaments) are anchored by specialized, periodically repeating structures known as telophragm and mesophragm. Telophragms are formed by protein molecules oriented transversely across the muscle fiber and attached to the sarcolemma (the fiber membrane). In a Longitudinal section of a muscle fiber, telophragms appear as dark transverse lines about 100 nm thick, referred to as Z-lines. Midway between two adjacent telophragms lies another transverse structure, the mesophragm, which is called the M-line in longitudinal sections.

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Fig. 12. Striated (skeletal) muscle tissue:

1 — muscle fiber; 2 — sarcolemma; 3 — myofibrils; 4 — nuclei

Thin (5 nm) actin filaments extend from the mesophragm toward the telophragm. Toward these filaments run thick (10 nm) myosin filaments originating from the telophragm and interdigitating with the actin filaments.

The segment between two Z-lines (telophragms) is called a sarcomere, which serves as the structural and functional unit of a myofibril. The region of the myofibril occupied by the mesophragm (M-line) with myosin filaments (myofilaments) extending from it in both directions is designated as the H-band (light zone). The region of the myofibril containing both myosin and actin filaments is the A-band (A-disc). The portions of two connected sarcomeres occupied by the Z-line (telophragm) with actin filaments extending from it in both directions form the I-band (I-disc).

The alternation of dark A-discs and light I-discs, aligned at the same level in adjacent myofibrils, creates the impression of transverse striation in histological preparations of skeletal muscle. At the level of the telophragm, the sarcolemma forms deep invaginations containing transverse tubules (T-tubules) of the agranular Endoplasmic reticulum, which branch out between the myofibrils of the muscle fiber.

Muscle contraction is driven by interactions between Actin and myosin. During muscle contraction, actin myofilaments slide toward the myosin myofilaments. During muscle relaxation, the myofilaments move in opposite directions. Throughout this process, the length of the A-discs remains unchanged, whereas the I-disc decreases in size.

Based on the number of myofibrils in the sarcoplasm, muscle fibers are classified into slow ("red") fibers, which contain few myofibrils and abundant sarcoplasm, and fast ("white") fibers, which contain many myofibrils and little sarcoplasm. "Red" muscle fibers contract slowly but can sustain activity for a long time. "White" muscle fibers contract rapidly and fatigue quickly. The combination of Slow and fast striated muscle fibers in muscles ensures both rapid reaction (contraction) times and prolonged working capacity.

The developmental source of striated (skeletal) muscle tissue is the myotome Cells of the somites. During early embryonic development, single-nucleated spindle-shaped cells—myoblasts—migrate from the mesodermal myotomes. Multiplying rapidly, myoblasts form the primordia of future muscles in appropriate locations. Rapid nuclear division leads to the loss of cellular boundaries in myoblasts as they transform into large multinucleated complexes known as muscle fibers. Within developing muscle fibers, the number of myofibrils increases, and transverse striation appears. During the second half of intrauterine development and postnatal ontogeny, muscle fibers grow in length and thickness through an increase in their myofibril content. Simultaneously with the growth and differentiation of muscle fibers, they fuse with satellite cells. Satellite cells reside beneath the sarcolemma of muscle fibers and serve as a source for new fibers; they are capable of dividing and giving rise to myoblasts following muscle injury.

Non-striated (smooth) muscle tissue forms the contractile apparatus in the walls of Internal Organs, glandular ducts, Blood and Lymphatic vessels, and other organs. The structural element of this tissue is the smooth muscle Cell (myocyte). Smooth myocytes are spindle-shaped cells measuring 20—500 µm in length and 5—8 µm in thickness. Each myocyte possesses a single rod-shaped Nucleus located in the center of The Cell. During myocyte contraction, The Nucleus bends or even becomes spirally twisted. Organelles, including numerous Mitochondria, are concentrated near the poles of the cell. The Endoplasmic reticulum and Golgi apparatus are poorly developed, indicating a low synthetic activity in myocytes. The cytoplasm of myocytes abounds in actin and myosin fibrils arranged at an angle to one another rather than in parallel. The proportion of actin (compared to myosin) is higher in smooth myocytes than in striated muscle fibers. The interaction of actin and myosin myofilaments occurs via a sliding mechanism, though it operates differently than in skeletal muscle tissue. Smooth myocytes lack transverse striations, contract independently of conscious will, and their functions are controlled by the Autonomic (vegetative) Nervous system. Smooth myocytes aggregate into bundles reinforced by fine collagen and elastic fibers.

Cardiac Striated muscle tissue is formed by closely apposed muscle cells with cross-striations, known as cardiomyocytes. At the same time, cardiac muscle cells contract automatically, governed by the rhythm of The Heart's conduction system and the Influence of the autonomic (vegetative) nervous system. Cardiomyocytes are elongated cells (up to 100—150 µm long) with a thickness of 10—20 µm, each containing a centrally located nucleus. General-purpose organelles are situated at the cell poles. Mitochondria are arranged in chains along the myofibrils. Cardiomyocytes contain inclusions such as glycogen and Lipids. The arrangement of actin and myosin myofibrils in cardiomyocytes mirrors that in skeletal muscle cells. Thin actin myofibrils are anchored at one end to the telophragm, forming the Z-line. Thick (myosin) myofibrils, interspersed between the actin filaments, are anchored at one end to the mesophragm (M-line) and directed toward the telophragm at the other.

Cardiomyocytes contact one another to form an integrated structural and functional contractile system. At the boundaries between adjacent cardiomyocytes lie intercalated discs, composed of contacting areas of the cytolemma of neighboring cells that resemble expanded desmosomes. Intercalated discs firmly bind neighboring cardiomyocytes together while enabling the rapid propagation of nerve impulses through them, allowing all cardiac myocytes to contract simultaneously. Through intercalated discs, cardiomyocytes are integrated not only structurally but also functionally into a cohesive heart muscle (myocardium).